SUN-EARTH-MOON SYSTEM VISUALIZER
I built these canvases to answer my kids' questions.
- Why does the Moon always show the same face?
- How can the Sun and Moon be up at the same time?
- Why are eclipses rare?
01 // Tidal locking
You are the Earth at the center of this canvas. A coin rolling on its edge around you with its face toward you shows only heads, and the Moon moves the same way.
Under tidal locking the Moon spins once per orbit, so one face always points at Earth; Earth's gravity pulled the spin into step over billions of years. With the lock off the Moon keeps a fixed orientation and Earth sees every side.
02 // Sun and Moon in one sky
The small panel shows your sky through one day. Pick a Moon phase to see when the Sun and Moon are both up.
The Moon reflects sunlight and does not glow. The half facing the Sun is always lit, and the phase is how much of that half Earth sees. For most of the month the Moon is off to one side and often up in daylight. A first quarter Moon rises around midday; a full Moon, opposite the Sun, is up mostly at night.
Distances are not to scale; the real Moon is about thirty Earth-widths away. Periods, day/night shading and phase geometry are accurate.
03 // Tilted orbit and eclipses
The Moon's orbit is tilted about 5° from the ecliptic, the plane of Earth's orbit, so most New and Full Moons pass above or below the Sun's line. A solar eclipse needs a New Moon at a node, one of the two points where the orbit crosses the ecliptic, and a lunar eclipse needs a Full Moon there. Those line up only in two eclipse seasons a year, so eclipses are rare.
The tilt is drawn much steeper than 5° so it is visible, but eclipse detection uses the real angle. With orbital tilt off the orbit is flat, so every New and Full Moon is an eclipse.
Sources:
· Orbital periods (sidereal year 365.256 d, sidereal month 27.322 d): NASA NSSDCA Moon Fact Sheet and Earth Fact Sheet.
· Tidal locking and phases: NASA Science, Earth's Moon and Moon Phases.
· Orbital inclination (5.14°) and eclipses: NASA NSSDCA Moon Fact Sheet (above) and NASA Science, Eclipses.